The Kindle, and especially the Kindle DX, is the best product out there as a book reader but is limited for aviation and most other use by not supporting directories ("folders" in windoze-speak).
This makes it very hard to browse large numbers of files e.g. PDFs of approach plates.
The Ipad does a much better job of this but has two issues: it cannot be dimmed sufficiently for night flight, and it suffers badly from reflections within the cockpit if flying in sunshine.